Frequency modulation is widely used today in the transmission of message signals, due principally to its noise suppression properties. Frequency modulation refers to the process whereby the frequency of a carrier signal is modulated by a message signal--the amplitude of the carrier signal is intended to remain constant. The information in FM signals is thus contained in the zero crossings of the modulated signal which are detected by a receiver. European commercial stereo broadcast FM is radiated in a band extending from 88 to 108 MHz.
FM receivers are often of the heterodyne type, in which incoming signals received by the receiver are mixed with a signal from a local oscillator, so as to reduce the frequency of the incoming signal and thereby enable better filtering thereof.
The frequency of the local oscillator can be controlled with respect to the carrier frequency so that the variable radio frequency of each signal channel may be converted to a fixed intermediate frequency. The modulated intermediate frequency signal can then be detected by appropriate demodulation circuitry.
FIG. 1 shows an example of a down-conversion superheterodyne FM receiver 1 which comprises essentially an antenna 2, a radio frequency (RF) stage 3, a mixer 4, a local oscillator 5, an intermediate-frequency (IF) filter 6, a amplifier/limiter 7, a demodulator 8, an automatic frequency control (AFC) stage 9, an audio amplifier 10 and a speaker 11. The antenna 2 captures signals within a range including the 88-108 MHz FM transmission band and transmits corresponding electrical signals to the RF stage 3.
The RF stage 3 is tuned to the carrier frequency f.sub.c of a desired signal channel and amplifies the electrical signals received from the antenna 2 within a frequency bandwidth centered on the frequency f.sub.c, whilst rejecting unwanted image frequencies. The local oscillator signal and the RF stage signal are multiplied by the mixer 4, so that the message signal at the output of the mixer 4 is modulated at the intermediate frequency f.sub.IF.
The tuning of the local oscillator 5 and the RF stage 3 is coupled so as to improve the image frequency rejection of the FM receiver 1. The operating characteristics of the IF filter 6 are chosen so that only signals within a bandwidth centered around the intermediate frequency f.sub.IF are passed.
The amplifier/limiter 7 limits the amplitude of the signal passed by the IF stage 6 in order to remove any AM component therefrom.
The FM demodulator 8 demodulates the signal from the amplifier/limiter 7 and passes the message signals thus extracted to the audio amplifier 10. Signals are then sent from the output of the audio amplifier 10 to the speaker 11 for reproduction. Finally, the AFC stage 9 fine tunes the frequency f.sub.LO of the local oscillator signal according to the DC output of the demodulator 8 in order to keep the frequency IF constant.
In many FM receivers, the intermediate frequency f.sub.IF is chosen to be about 10.7 MHz, which is slightly larger than one-half of the FM broadcast band of 20 MHz, so that all image frequencies lie outside the broadcasting band. Superheterodyne FM receivers which use such high intermediate frequencies may be largely integrated in a single integrated circuit, however the signal processing involved in such receivers necessitates the use of off-chip tuned LC circuits, and ceramic or surface acoustic wave (SAW) filters in the RF, heterodyning and demodulation stages of the receiver.
The use of lower intermediate frequencies that can be tuned by fully integrated low Q filters removes the need for these off-chip filters, thereby enabling the production of a single chip FM receiver which uses only a minimum of external components.
Consequently, some existing FM receiver designs thus use an intermediate frequency f.sub.IF of 70 kHz and can be implemented substantially as single integrated circuits. This intermediate frequency value is chosen so that the image frequency of a selected signal channel occurs about halfway between the center frequency of that signal channel and that of the adjacent channel.
However, a shortcoming in such FM receivers is their reduced bandwidth. Aliasing causes severe distortion when the modulation frequency, that is, the frequency of the message signal, exceeds the intermediate frequency f.sub.IF. Moreover, offsets in multipliers and edge detectors used in demodulators result in the occurrence of severe aliasing distortion when the modulation frequency exceeds half the intermediate frequency. The reduction in the intermediate frequency f.sub.IF to 70 kHz results therefore in a corresponding reduction in the bandwidth of the message signal which can be received and demodulated by the FM receiver.